Effect of Anisotropy Constants on Squareness Ratio in Sr–La–Co High-Performance Sintered Ferrite Magnets at Elevated Temperatures

To decrease the irreversible demagnetization of Sr-La-Co sintered ferrite magnets at elevated temperatures, it is required to remarkably improve the squareness ratio (Hk/HcJ), where Hkand HcJare the knee field and coercivity, respectively. It is very important to find out the main reason behind the effects on Hk/HcJat high temperature. Particular attention has been paid to the effect of the ratio of anisotropy constant (Ku2/Ku1) on Hk/HcJfor Sr1-xLaxFe12-xCoxO19(x = 0-0.4) sintered ferrite magnets above room temperature, where Ku2, Ku1, and x are the quartic and quadratic anisotropy constants, and the substituted amount, respectively. The value of Hk/HcJlinearly decreases as x value increases at every temperature. Furthermore, it decreases with higher temperatures. The value of Ku2/Ku1remarkably increases with increasing x value at elevated temperatures. The value of Hk/HcJlinearly decreases in the order of x value as Ku2/Ku1increases for all the magnets above room temperature. Uniaxial anisotropy term (Ku1+ Ku2) has an effect on the orientation in the direction of easy magnetization. However, biaxial anisotropy term (Ku2/2) has two different directions of easy magnetization. The difference in easy direction of biaxial anisotropy makes an angle of π/4 to each other. An excessive increase in Ku2/Ku1is not necessarily effective to increase Hk/HcJ. The decrease in Hk/HcJis mainly caused by the increase in Ku2/Ku1at elevated temperatures.